When the Giants line up Sunday to open their season against the Rams, 20 of their 22 starters will return to the roles they filled a year ago. That was the plan heading into the summer and throughout training camp, a plan that crystallized the team’s obvious goal of improving depth.

Did the Giants succeed? In some cases, yes. In others, no.

“In some spots, we’re not very deep,” Jim Fassel said, “and if we get many injuries we could be playing with some very inexperienced people.”

All it took was a glance at the expressions on the faces of some of the more improbable Giants left standing to recognize where the team is strongest and where it is vulnerable.

Rookies Jeff Roehl and Wayne Lucier on Saturday were beaming after hearing they had made the team, a sure sign the offensive line is as thin as the Giants’ chances without a healthy Kerry Collins.

“It’s pretty overwhelming so far, in a good way, obviously,” said Roehl, a street free agent from Northwestern. “I woke up and wanted to come in and see what was going to happen, a little bit nervous. I could tell right away things were looking good because I saw a couple guys on the O-line packing up and it wasn’t me. Things can change any minute and I have to keep proving that I should be here.”

The Giants hope Roehl fades into the background from here on out and spends his first NFL season learning the ropes on the practice field. Sometimes the best depth goes unused. Just how deep is this Giants team? Here’s a closer look:

QUARTERBACK: Heaven forbid Kerry Collins gets hurt. Jesse Palmer is the backup by default, providing little evidence he can run the show. Jason Garrett knows exactly what to do but doing it is an entirely different matter.

RUNNING BACK: No one around the Giants wants to consider life without Tiki Barber, but there’s enviable options here. Dorsey Levens is tough, although he probably can’t handle full-time work. Delvin Joyce has a spark and Brian Mitchell can fill in in an emergency. Oh, yes, there’s also Ron Dayne, who could go from inactive to starter in a moment’s notice.

RECEIVER: After starters Amani Toomer and Ike Hilliard, the other three receivers (Tim Carter and raw rookies David Tyree and Willie Ponder) have a total of two NFL receptions. Carter looks like a stud, though.

TIGHT END: Marcellus Rivers starts in two tight end sets and he’s a capable receiver and incapable blocker. Rookie Visanthe Shiancoe is a promising prospect who at present is too green for regular use. No one here replaces Dan Campbell’s toughness.

DEFENSIVE LINE: At long last, quality behind the starters. Keith Washington is a fine alternative at end and first-round pick William Joseph will be a factor at tackle. Second-round pick Osi Umenyiora should contribute immediately as a pass-rusher.

DEFENSIVE BACKS: Giants are one injury away from having Ralph Brown as a starting cornerback, not in inviting scenario. Safety depth appears solid with Johnnie Harris and Ryan Clark, and at least there’s experience at corner with Ray Green and Kato Serwanga. Best of the bunch at corner, in time, might be Frank Walker.